Soccer is one of the most popular sports in the U.S. with 17.7 million youth players reported in 2005. Girls are participating in this sport in increasing numbers. Although several studies have documented that female youth soccer players have a higher incidence of injury compared to male youth players, little research has evaluated the risk factors that are associated with the increased risk of injury among girls. A prospective study of this young population is needed to determine injury incidence rates of soccer players as well as risk factors that predispose some girls to soccer injuries. We propose to perform a four-year, prospective, cohort study of elite soccer players in the Seattle area. Our specific aims are to 1) determine the incidence rate of injuries among female youth soccer players aged 12 to 14 years, 2) prospectively evaluate hip and knee muscle strength and jump biomechanics, and 3) evaluate the type of soccer playing surface as a potential extrinsic risk factor. At the beginning of the soccer season, all subjects will be evaluated by physical therapists for hip flexion, extension, abduction, and adduction muscle strength, hamstring and quadriceps muscle strength, and jump biomechanics as potential injury risk factors. Subjects will be followed for the entire 12- month soccer season for the occurrence of soccer-related injuries. We will use an email injury surveillance system to determine soccer injury incidence rates as well as to collect data on the type and location of injury and amount of time lost from soccer participation. We will also collect detailed soccer playing time information for soccer practice and game sessions. We will perform a nested case control study to evaluate time-dependent risk factors such as type of playing surface, using two control (uninjured) subjects to one case subject. Considering the growing population of young female soccer players and their risk of soccer-related injuries, a large number of injuries are projected in this population. When the aims of this grant are achieved, we will have gained a more complete understanding of the hypothesized risk factors that will be useful in the development of injury prevention strategies. Soccer is one of the most popular youth sports, with girls participating in increasing numbers. Little research has evaluated risk factors among female youth players. This project will evaluate lower extremity muscle strength and jump biomechanics as potential risk factors for soccer injuries in this vulnerable population.